There have been many different solutions to the problem of the timing in four-stroke-cycle reciprocating engines. One in particular that has been implemented in practice and is widespread is where a number of mushroom type valves, driven by an eccentric, regulate the suction and exhaust phases of the cycle developed by the engine.
Between the eccentric and the valves, there are other mechanical components making up a particular kinematic chain such as: caps, rods, rocking arms, timing devices, rockers, etc.
The eccentric rotates at a speed which is half that of the driven shaft; it is therefore necessary to provide an appropriate system of speed reduction.
Although expanding technology and experience has allowed higher degrees of efficiency and reliability, the results are complex, noisy and inefficient, and subject to wear and breakdowns; they require constant maintenance with considerable high costs of production.